Non-resonant oven cavity and resonant antenna system for microwave heating oven



Nov. 30, 1965 NON-RESONANT OVEN CAVITY AND RESONANT ANTENNA SYSTEM FORMICROWAVE HEATING OVEN Filed July 2 J. E. STAATS 2 Sheets-Sheet 1INVENTOR.

JAMES E. STAATS BY ewwm W f 7' 6 ATTYS.

Nov. 30, 1965 J. E. STAATS 3,221,132

NON-RESONANT OVEN CAVITY AND RESONANT ANTENNA SYSTEM FOR MICROWAVEHEATING OVEN Filed July 22, 1963 2 Sheets-Sheet 2 INVENTOR.

JAMES E. STA/1T8 ATTYS.

United States Patent 3,221,132 N ON-RESONANT OVEN CAVITY AND RESONANTANTENNA SYSTEM FOR MICROWAVE HEAT- ING OVEN James E. Staats, Louisville,Ky., assignor to General Electric Company, a corporation of New YorkFiled July 22, 1963, Ser. No. 296,622 16 Claims. (Cl. 219-1055) Thepresent invention relates to microwave heating ovens, and moreparticularly to non-resonant oven cavities and resonant antenna systemstherefor.

In the co-pending application of James E. Staats, Serial No. 181,144,filed March 20, 1962, there is disclosed a control and power supplysystem for a multiple cavity magnetron device that is especiallyconstructed and arranged to achieve stable and efficient operation atanodecathode voltages of relatively low amplitude, thereby rendering themagnetron device particularly well suited for incorporation inelectronic cooking apparatus designed for home use. More particularly,this magnetron device is designed to oscillate at an ultra-highfrequency of about 915 mc., employing anode-cathode voltages in thegeneral range of 250 to 1000 volts D.C., with corresponding RF. poweroutputs in the general range of 80 to 2400 watts. Specifically, thecircuit of the above application Serial No. 181,144 is particularlydesigned to operate from the household A.C. supply source having afrequency of about 60 cycles and an R.M.S. voltage in the general rangeof 220 to 250 volts, producing a plate voltage of about 570 volts D.C.,thus providing continuous R.F. power output of about 700 watts at theultra-high frequency of 915 me. The microwave heating oven of thepresent invention is particularly adapted for use with such a magnetronand power supply system therefor operating under the conditions noted.

In the microwave heating ovens common in the art heretofore, thedimensions of the object to be heated and the dimensions of theenclosure or oven cavity for receiving the object to be heated have beengenerally comparable to the wavelength of the microwave energy utilizedor multiples thereof, and as a result there is always present thepossibility of having one or more resonant modes excited or existing inthe oven cavity. The perturbations of the resonant modes caused by thedielectric properties of the object to be heated result in nonuniformheating; and accordingly, it has been customary heretofore to utilizeso-called mode stirring devices in an effort to counteract theperturbations of the resonant modes caused by the dielectric objects tobe heated, but the mode stirring devices in turn may create undesiredmodes resulting in non-uniform heating.

Accordingly, it is a general object of the present invention to providean oven cavity and an antenna therefor in a microwave heating oven ofthe type set forth, wherein the oven cavity is non-resonant and free ofresonant modes during the operation thereof.

Another object of the invention is to provide a microwave heating ovenof the type set forth in which the dimensions of the oven cavity arerelated to the wavelength of the microwave energy used in the ovencavity so that the oven cavity is free of resonant modes at thewavelength of the microwave energy.

Another object of the invention is to provide a microwave heating ovenof the type set forth which quickly and uniformly heats a wide varietyof objects having a wide range of dielectric characteristics.

Another object of the invention is to provide a microwave heating ovenof the type set forth utilizing a source of microwave energy operatingat the authorized frequency of 915 mc., wherein the oven cavity isbox-like with an X dimension of the same order of magnitude as thewavelength of the microwave energy and with a Y dimension substantiallyequal to the X dimension and with a Z dimension not greater thanone-half the X dimension, so as to provide an oven cavity that will benon-resonant at the authorized frequency.

Another object of the invention is to provide an improved microwaveheating oven incorporating in the nonresonant oven cavity an improvedresonant antenna structure.

Another object of the invention is to provide in a microwave heatingoven of the type set forth, a box-like oven cavity having an antennaarranged therein adjacent to and below the top wall and cooperatingtherewith to provide a radiating device of the plane reflector arraytype, the antenna being spaced from the top wall a distance to provide amaximum gain.

In connection with the foregoing object, it is another object of thepresent invention to provide in combination, a box-like oven cavityhaving an antenna disposed therein adjacent to the top wall thereof andspaced from the cooking surface a distance of at least one-quarterwavelength of the operating frequency of the microwave energy used inthe oven cavity in order to obtain a uniform field at the cookingsurface.

A further object of the invention is to provide in the oven cavity of amicrowave heating oven of the type set forth, an antenna comprising twoor more dipole elements disposed to establish two or more field patternsrelated in phase such that the vector field summation is substantiallyuniform over an area sufliciently large to encompass the object to beheated.

A further object of the invention is to provide in a box-like ovencavity for a microwave heating oven of the type set forth, an antennaincluding two substantially symmetrical arm structures with anelectrical input terminal therebetween, each of the arm structures beingof branched configuration and constituting one or more dipole elements.

A still further object of the invention is to provide in a microwaveheating oven of the type set forth, a box-like non-resonant oven cavityhaving an antenna therein disposed substantially in a plane positionadjacent to and below and substantially parallel with the top wall, theantenna including two substantially symmetrical antenna membersextending between the side walls and having physical lengths shorterthan the wavelength of the microwave energy supplied to the oven cavity,the antenna members being reactively loaded to impart thereto anelectrical length equivalent to the wavelength of the microwave energyused.

Further features of the invention pertain to the particular arrangementof the elements of the microwave heating oven and of the oven cavity andantenna forming a part thereof, whereby the above-outlined andadditional operating features thereof are attained.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification, taken inconnection with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a microwave heating oven incorporatingtherein an oven cavity and an antenna embodying the present invention;

FIG. 2 is an enlarged longitudinal vertical sectional view through theoven cavity along the line 22 of FIG. 1;

FIG. 3 is an enlarged lateral vertical sectional view through the ovencavity along the line 3 3 oflfIQ Zg,

FIG. 4 is an enlarged horizontal section through the oven cavity alongthe line 4-4 of FIG. 3; and

FIG. 5 is a further enlarged vertical sectional view of an upper cornerof the oven cavity along the line 55 of FIG. 4.

Referring now to FIG. 1 of the drawings, the microwave heating oventhere illustrated, and embodying the features of the present invention,is in the form of an electronic heating oven specially designed for homeuse. More particularly, the microwave heating oven 10 comprises anupstanding substantially box-like wall structure 11 formed of steel anddefining an oven cavity therein, the wall structure 11 havingsubstantially the configuration of a simple tetragonal, and moreparticularly an orthohexahedron. Specifically, the oven cavity isdefined by a top wall 12, a bottom wall 13, a pair of opposed side walls14 and 15, a front wall 16, and a rear wall 17, the front wall 16 havinga doorway 18 therein opening into the cooking cavity defined by the wallstructure 11. Further, the front wall 16 is provided with a front door20 hingedly connected thereto by associated hinge structure 21 andcooperating with the doorway 18, whereby the door 20 is movable betweena substantially vertical closed position and a substantially horizontalopen posi tion with respect to the doorway 18 in the wall structure 11.Also the front door 20 includes a handle 22 and the usual innerforaminous metal sheet 23 formed of steel which cooperates with the wallstructure 11 completely to enclose the oven cavity when the front door21) occupies its closed position.

An antenna structure generally designated by the numeral 30 is arrangedwithin the oven cavity defined by the wall structure 11 and is disposedadjacent to and immediately below the top wall 12. Referringparticularly to FIG. 4, the antenna structure 30 includes twotransversely extending spaced-apart antenna members 31 and 41 whichextend the width of the oven cavity and have the outer ends thereoffixedly secured to the adjacent side walls 14 and 15, respectively, theantenna members 31 and 41 being formed of an electrically conductivematerial, such as aluminum. More specifically, the antenna member 31 isformed as an elongated flat strip extending the width of the oven cavityand having a first upturned flange 32 on one end thereof and a secondupturned flange 33 on the other end thereof, the flanges 32 and 33extending in the same direction from the antenna member 31 and beingdisposed substantially perpendicular thereto and parallel to each other;the flanges 32 and 33 are provided with openings therethrough to receivefasteners, such as the screws 34, for mounting the antenna member 31upon the side walls 14 and 15, respectively. The other antenna member 41likewise constitutes a narrow strip of metal provided at the oppositeends with upturned flanges 42 and 43, respectively, the flanges 42 and43 extending in the same direction from the antenna member 41 and beingdisposed substantially perpendicular thereto and substantially parallelto each other; the flanges 42 and 43 are provided with openingstherethrough to receive fasteners such as the screws 44, for mountingthe antenna member 41 upon the side walls 14 and 15, respectively. Twoconnecting members 51 and 61 are provided to interconnect spaced pointsof the antenna members 31 and 41, the connecting members 51 and 61 beingalso formed of an electrically conductive material, preferably aluminummetal, and having a length equal to the distance between the outer edgesof the antenna members 31 and 41 and having widths equal substantiallyto the widths of the antenna members 31 and 41. More particularly, theconnecting member 51 is disposed toward the side wall 14 and arrangedwith the axis thereof generally parallel to the side wall 14, the outerends of the connecting member 51 lying upon the antenna members 31 and41, whereby the ends of the connecting member 51 overlie intermediateportions of the antenna members 31 and 41. Aligned openings are formedin the overlapping portions of the connecting member 51 and the antennamembers 31 and 41 for the reception therethrough of fasteners, such asthe screws 57 (see FIG. 5 also) that serve to interconnect the antennamembers 31, 41 and 51. The connecting member 61 is disposed toward theside wall 15 and arranged with the axis thereof generally parallel tothe side wall 15, the outer ends of the connecting member 61 beingdisposed respectively upon intermediate portions of the antenna members31 and 41. Aligned openings are formed in the overlapping portions ofthe antenna members 31, 41 and 61 for the reception therethrough offasteners, such as the screws 67 (see FIG. 3 also). An antenna feedmember 71 is provided interconnecting the midpoints of the connectingmembers 51 and 61, the feed member 71 having substantially the samewidth and thickness as the antenna members 31, 41, 51 and 61 and beingdisposed below the connecting members 51 and 61 and in substantially thesame plane as the antenna members 31 and 41, the longitudinal axis ofthe feed member 71 being substantially parallel to the rear wall 17 andto the axes of the antenna members 31 and 41. The ends of the feedmember 71 extend beneath the mid-portions of the connecting members 51and 61 and aligned openings are provided through the ends of the feedmember 71 and the associated connecting members 51 and 61, the alignedopenings receiving suitable fasteners therethrough, such as screws, tointerconnect the members 51, 61 and 71. I

Connection is also made between the antenna 30 and the top wall 12, fourconductive connections being provided and four non-conductive orinsulated connections being provided. A first conductive connection ismade between the midpoint of the antenna member 31 and the top wall 12by means of a conductive slug 35 formed of any suitable electricallyconductive material such, for example, a aluminum. The lower end of theslug 35 has a tapped opening therein in alignment with an opening in theantenna member 31 and adapted to receive a suitable fastener, such asthe screw 37, therein (see particularly FIG. 5). The upper end of theslug 35 also is provided with a tapped opening and is positioned inalignment with an opening in the top wall 12 for the reception of asuitable fastener, such as the screw 36, therein. A like conductiveconnection is made between the midpoint of the antenna member 41 and thetop wall 12 by an electrically conductive slug 45 having the lower endthereof secured to and in electrical connection with the antenna member41 and having the upper end thereof secured to and in electricalconnection with the top wall 12. A similar conductive connection is madebetween the midpoint of the connecting member 51 and the top wall 12 byan electrically conductive slug 55 having the lower end thereof securedto and in electrical connection with the connecting member 51 and theunderlying end of the feed member 71, and having the upper end thereofsecured to and in electrical connection with the top Wall 12. A likeconductive connection is made between the midpoint of the connectingmember 61 and the top wall 12 by an electrically conductive slug 65having the lower end thereof secured to and in electrical connectionwith the connecting member 61 and the underlying adjacent end of thefeed member 71 and having the upper end thereof secured to and inelectrical connection with the top wall 12.

The insulated connections between the antenna 30 and the top wall 12 aremade by four insulators 52, 53, 62 and 63, all formed of a suitableelectric insulating material such as the ceramic Steatite. Morespecifically, the insulator 52 has a threaded opening in the lower endthereof and in alignment with openings in the antenna member 31 and theconnecting member 51 for the reception therein and therethrough of asuitable fastener, such as the screw 57 (see FIG. 5 also); the upper endof the insulator 52 is likewise provided with a threaded opening whichis in alignment with the opening in the top wall 12 for the receptiontherein and therethrough of a suitable fastener, such as the screw 54,for mounting the insulator 52 and the connected parts upon the top wall12. The insulator 53 is constructed and mounted like the insulator 52and interconnects the overlying portions of the antenna member 41 andthe connecting member 51 to the top wall 12. The insulator 62 also has aconstruction and is mounted like the insulator 52 and serves tointerconnect the overlying portions of the antenna member 31 and theconnecting member 61 to the top wall 12. The insulator 63 is likewiseconstructed and mounted in the same manner as the insulator 52 andserves to connect the overlying portions of the antenna 41 and theconnecting member 61 to the top wall 12.

The microwave energy used in operating the microwave heating oven 10,more particularly, the energy coupled to the antenna 30 described above,is derived from a suitable source that is diagrammatically illustratedat 90 in FIG. 1 of the drawing. One preferred source of microwave energyis a crossed-field electric discharge device in the form of a multiplecavity magnetron device of the type illustrated and described in theco-pending application of James E. Staats, Serial No. 105, 983, filedApril 27, 1961. The magnetron device described in that application isparticularly well suited for incorporation in electrical cookingapparatus such as the microwave heating oven of the present invention.More particularly, this magnetron device is designed to oscillate at anultra-high frequency of about 915 mc., employing anode-cathode voltagesin the general range 250 to 1000 volts DC, with corresponding R.F. poweroutputs in the general range 80 to 2400 watts. Specifically at a platevoltage of about 290 volts DC, this magnetron device has a continuousR.F. power output of about 100 watts at the ultra-high frequency of 915mc.; at a plate voltage of about 570 volts D.C., this magnetron devicehas a continuous R.F. power output of about 700 watts at the ultra-highfrequency of 915 mc.; and at a plate voltage of about 1000 volts DC,this device has a peak R.F. power output of about 2400 watts at theultra-high frequency of 915 me.

By using the control and power supply systems illustrated and describedin the copending application of I ames E. Staats, Serial No. 181,144,the above-described magnetron device can be readily operated from theusual household 3-wire Edison network of 236 volts, singlephase,60-cycles, A.C. More specifically, the power supply system disclosed inthis application includes a voltage doubler and rectifier circuit whichproduces from the 236 volts A.C. of the Edison network the relativelyhigh D.C. output voltage of 570 volts that can be supplied to themagnetron device which oscillates to supply ultrahigh frequency energyat about 915 mc., the R.F. power output of the magnetron device beingapproximately 700 watts.

The microwave energy so generated is conducted from the source 90 to theantenna 30 within the wall structure 11 by means of the transmissionline 80. Preferably the transmission line 80 is of the generalconstruction and arrangement of that disclosed in the copendingapplication of Louis H. Fitzmayer, Serial No. 135,582, filed September1, 1961, now Patent No. 3,172,987, granted March 9, 1965; whereby thetransmission line 80 includes an outer conductor 81 and an innerconductor 82 (see FIG. 2) which connects with a terminal 83 that isconnected to an electrical input terminal 72 mechanically secured to andin electrical connection with the midpoint of the feed member 71 (seeFIG. 4).

An important feature of the present invention resides in the fact thatthe microwave energy is introduced into a non-resonant cooking cavity bymeans of a plurality of resonant dipole antenna elements coupled in theproper phase relationship for establishing a uniform composite field inthe non-resonant cooking cavity. An important advantage of using thenon-resonant cooking cavity is derived from the fact that theintroduction of dielectric materials to be heated into the cookingcavity does not affect 6 its resonance, and therefore minimum fielddistortion and decoupling eifects are encountered. As a result, themicrowave heating oven of the present invention can efficiently heat awide variety of dielectric materials all without the need for resortingto complicated and relatively inefiicient mode stirring structures.

The cooking cavity defined by the conductive wall structure 11 isnon-resonant in the sense that the dimensions of the oven cavity arerelated to the wavelength of the microwave energy from the generator sothat the oven cavity is free of resonant modes at the operatingwavelength. When the cooking cavity is box-like in shape, andspecifically, when the cooking cavity is an orthohexahedron, asillustrated in the drawing, the resonant wavelength thereof can bedirectly calculated from the equa- In accordance with the presentinvention the dimensions X, Y and Z are chosen so that when they aresubstituted in Equation 1 above, none of the resonant wavelengths of thecorresponding cooking cavity correspond to the wavelength of themicrowave energy source 90; furthermore, the presence of an object to beheated within the cooking cavity has a pulling effect, whereby it isdesirable that the operating frequency of the microwave energy source 90should be displaced from the nearest resonant frequency of the cookingcavity about 5% of the wavelength of the microwave energy from thesource 90. In other words, the cooking cavity may have any combinationof dimensions X, Y and Z except those which possess resonant wavelengthswithin 5% of the frequency of the microwave generator 90. Cookingcavities having dimensions substantially larger than the wavelength ofthe source 90 tend to posess a large number of resonant frequencies ormodes which are very close together, i.e., separated by less than 10% ofthe wavelengths involved, whereby it is further desirable that the X andY dimensions be the same order of magnitude as the wavelength of themicrowave energy source 90. One particular illustration of a combinationof dimensions useful in accordance with the present invention and beingnon-resonant is obtained when: the X dimension is of the same order ofmagnitude as the wavelength of the microwave energy from the source 90,the Y dimension is substantially equal to the X dimension, and the Zdimension is not greater than one-half the X dimension.

The antenna 30 illustrated in the drawing is of the plane reflectorarray type and the maximum gain is obtained when the distance betweenthe antenna and the top wall 12, i.e., the distance D in FIG. 5, iswithin the range from about 5% to 15% of the wavelength of the microwaveenergy from the source 90, the preferred spacing being about 10% of thewavelength of the source 90 or slightly less. In order to obtain themost uniform field for heating and cooking purposes, the cooking surfaceshould be placed at least one-quarter wavelength from the antenna, andaccordingly, the distance H in FIG. 3 should be at least one-quarter ofthe wavelength of the source 90 so that the cooking surface which isdisposed a short distance upwardly from the bottom wall 13 will be at apoint of maximum uniformity of the heating field.

The antenna 30 as illustrated in FIG. 4 includes a plurality of resonantdipole elements all coupled in proper phase relationship forestablishing a uniform composite field within the cooking cavity. Morespecifically, as illustrated in FIG. 4, the cooking cavity isapproximately square and has X and Y dimensions substantially equivalentto a wavelength of the microwave energy source 90. The principal antennamembers 31 and 41 are each disposed one-quarter wavelength from theadjacent grounded wall, the antenna member 31 being placed parallel tothe rear wall 17 and spaced about one-quarter wavelength therefrom, andthe antenna member 31 being placed' parallel to the front wall 16 andspaced about one-quarter wavelength therefrom. The connecting members 51and 61 in turn are disposed one-quarter wavelength from the adjacentparallel grounded walls, i.e., the connecting member 51 being disposedparallel to the side wall 14 and spaced therefrom about one-quarterwavelength and the connecting member 61 being disposed parallel to theside wall 15 and spaced therefrom about one-quarter wavelength. Thelongitudinal axis of the feed member '71 is disposed along the linespaced one-half wavelength from the walls 16 and 17 and parallel theretoand the ends of the feed member 71 terminate at points spacedapproximately one-quarter wavelength from the adjacent side we walls 14and 15, respectively.

It will be seen therefore that the slug 35, for example, is disposedone-quarter wavelength from each of the insulators 52 and 62 which arein turn disposed one-quarter wavelength from the side walls 14 and 15respectively. The slug 45 is similarly positioned one-quarter wavelengthfrom each of the insulators 53 and 63 which are in turn positionedone-quarter wavelength from the side walls 14 and 15, respectively. Theslug 55 is positioned one-quarter wavelength from the input connection72 and onequarter wavelength from each of the insulators 52 and 53. Theslug 65 is likewise positioned one-quarter wavelength from the feedconnection 72 and one-quarter wavelength from each of the insulators 62and 63. The antenna 30 therefore is of branched configuration, issymmetrical with respect to the oven cavity and includes a plurality ofdiepole antenna elements each of which radiates into the non-resonantcooking cavity and in proper phase relationship to provide a uniformfield of microwave energy therein, the electric field beingdiagrammatically illustrated by the dashed lines in FIGS. 2 and 3 andindicated with the arrows marked with the letter B. The resultant fieldis substantially uniform throughout a large area disposed centrally ofthe cooking cavity and uniformly decreases to zero at the outer edges ofthe bottom wall 13.

As has been pointed out above, the X and Y dimensions of the cookingcavity are preferably not equal to one wavelength of the microwaveenergy from the source 90, and in the preferred embodiment of theinvention are actually slightly less than one wavelength of themicrowave energy source 90, whereby the antenna members 31 and 41 have aphysical length which is slightly less than a wavelength of the source90; and likewise, the connecting members 51 and 61 and the feed member71 have physical lengths that are slightly less than onehalf thewavelength of the source 90. The various dipole elements of the antenna30 are, however, reactively loaded, and more particularly capacitivelyloaded by the ceramic insulator-s 52, 53, 62, and 63 which have theeffect of electrically increasing the apparent length of the variousantenna members so that the various antenna dipole sections are resonantat the frequency of the microwave source 90 although physicallycontained within the wall structure 11 which has dimensions X and Y lessthan the wavelength of the microwave energy from the source 90.

In accordance with one preferred operative example of the improvedmicrowave heating oven of the present invention, the microwave energysource 90 operates at the governmentally assigned frequency of 915 me,corresponding to a wavelength of approximately 12.9 inches. The Xdimension of the cooking cavity is equal to the Y dimension and is 12inches. The Z dimension of the cooking cavity is exactly one-half of theX dimension, namely, 6 inches. Such an orthohexahedron cooking cavityhaving the dimensions X=12 inches, Y=12 inches, and 2:6 inches has aplurality of resonant wavelengths as calculated by Equation 1 above, thethree longest resonant wavelengths being 17.15", 11.12" and 10.6". Eachof these three resonant mode wavelengths are removed from the generatorwavelength of 12.9"

more than 5%, whereby the cooking cavity having the dimensions statedwill be non-resonant, i.e., will be free of resonant modes at theoperating wavelength of 12.9".

The antenna members 31 and 41 each have a length of 12 inches, i.e., alength slightly less than one wavelength of the source 90, a width of 1inch, and a thickness of 0.045 inch; the connecting members 51 and 61and the feed member 71 have lengths of 6% inches, widths of 1 inch andthicknesses of 0.045 inch. However, the capacitive loading of theantenna 30 by means of the four ceramic insulators 52, 53, 62 and 63provides an apparent electrical length for the various dipole antennaelements so that the dipole elements are resonant at the wavelength ofthe microwave energy from the source 90. Furthermore, the distance D inFIG. 5 is about 1.0 inch to obtain the maximum gain from the antenna 30,and the distance H in FIG. 3 is slightly less than 5.0 inches and is,accordingly, slightly greater than one-quarter wavelength of themicrowave energy from the source 90.

Consequently, the antenna 30 will be resonant at the wavelength of themicrowave energy from the source and will provide a uniform symmetricalpattern with the radiation from each dipole element in proper phaserelationship to provide a uniform heating field in the center of thecooking cavity, the uniform field extending throughout the greaterportion of the volume of the wall structure 11. The resonant antenna 30moreover, will feed energy to the non-resonant cooking cavity so thatthe microwave heating oven 10 can accept a wide variety of dielectricmaterials for heating therein with a minimum of distortion of theelectric field and a minimum of decoupling efifects. As a result, alarge amount of energy is quickly and uniformly transferred from thesource 90 to dielectric materials placed in the oven cavity so that, forexample, pre-cooked frozen dinners can be heated in six minutes andrefrigerated dinners can be heated in four minutes.

It will be understood from the above operative example that themicrowave heating oven is compact in size, can be readily operated fromthe standard household electric power supply, and heats evenly andconsequently with low operating cost a wide variety of dielectricmaterials.

In view of the foregoing, it is apparent that there has been provided animproved microwave heating oven which comprises a non-resonant cookingcavity, and also there has been provided in the non-resonant cookingcavity an antenna that is resonant at the wavelength of the microwaveenergy from the connected source.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. A microwave heating oven comprising conductive wall structuredefining an oven cavity and having a doorway therein communicating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, an antenna arrangedwithin said oven cavity, and a source of microwave energy adapted tooperate at a predetermined wavelength and operatively coupled to saidantenna, the dimensions of said oven cavity being related to saidpredetermined wavelength of the microwave energy of said source so thatsaid oven cavity is free of resonant modes at said predeterminedwavelength and so that the wavelengths of the resonant modes of saidoven cavity are displaced by at least about 5% from said predeterminedwavelength.

2. A microwave heating oven comprising conductive wall structuredefining an oven cavity and having a doorway therein communicating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, an antenna arrangedwithin said oven cavity, and a source of microwave energy operativelycoupled to said antenna, said oven cavity having substantially theconfiguration of a simple tetragonal, wherein the X dimension issubstantially equal to the Y dimension and wherein the Z dimension isnot greater than /2 the X dimension, wherein the X dimension is of thesame order of magnitude as the wavelength of the microwave energy ofsaid source, and wherein said dimensions have values such that said ovencavity is non-resonant at the wavelength of the microwave energy of saidsource.

3. A microwave heating oven comprising substantially box-like conductivewall structure including top and bottom walls and a pair of side wallsand front and rear walls, said wall structure defining on oven cavitytherein, one of said walls having a doorway therein communicating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, an antenna arrangedwithin said oven cavity adjacent to and below said top wall, saidantenna cooperating with said adjacent top wall to provide a radiatingdevice of the plane reflector array type, and a source of microwaveenergy operatively coupled to said antenna, the dimensions of said ovencavity being related to the wavelength of the microwave energy of saidsource so that said oven cavity is non-resonant at the wavelengththereof.

4. The microwave heating oven set forth in claim 3, wherein said antennais spaced from said top wall a distance equal to from about 5% to aboutof the wavelength of the microwave energy of said source.

5. The microwave heating oven set forth in claim 3, wherein said antennais spaced from said bottom wall a distance at least equal to 25% of thewavelength of the microwave energy of said source.

6. A microwave heating oven comprising substantially box-like conductivewall structure including top and bottom walls and a pair of side wallsand front and rear walls, said wall structure defining an oven cavitytherein, one of said Walls having a doorway therein communieating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, an antenna arrangedwithin said oven cavity adjacent to and below said top wall, saidantenna cooperating with said adjacent top wall to provide a radiatingdevice of the plane reflector array type, and a source of microwaveenergy operatively coupled to said antenna, said oven cavity havingsubstantially the configuration of an orthohexahedron, wherein the Xdimension is of the same order of magnitude as the wavelength of themicrowave energy of said source and is substantially equal to the Ydimension and wherein the Z dimension is not greater than /2 the Xdimension, and wherein said dimensions have values such that said ovencavity is non-resonant at the wavelength of the microwave energy of saidsource.

7. A microwave heating oven comprising substantially box-like conductivewall structure including top and bottom walls and a pair of side wallsand front and rear walls, said wall structure defining an oven cavitytherein, one of said walls having a doorway therein communicating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, an antenna arrangedwithin said oven cavity and disposed substantially in a plane positionedadjacent to and below and substantially parallel with said top wall,said antenna including two substantially symmetrical arm structures andan electrical input terminal therebetween, whereby said antennaconstitutes a dipole antenna and cooperates with said adjacent top wallto provide a radiating device of the plane reflector array type, and asource of microwave energy operatively coupled to said electrical inputterminal, the dimensions of said oven cavity being related to thewavelength of the microwave energy of said source so that said ovencavity is non-resonant.

8. The microwave heating oven set forth in claim 7, wherein saidelectrical input terminal is disposed below the central portion of saidtop wall, one of said arm structures is of branched configuration and ispositioned below one side of said top wall, and the other of said armstructures is of branched configuration and is positioned below theother side of said top wall.

9. The microwave heating oven set forth in claim 7, wherein each of saidarm structures includes an antenna section having a length equivalent toone-half of the wavelength of the microwave energy of said source and isdisposed from said electrical input terminal a distance equivalent toone-quarter wavelength of the microwave energy of said source.

10. A microwave heating oven comprising substantially box-likeconductive wall structure including top and bottom walls and a pair ofside walls and front and rear walls, said wall structure defining anoven cavity therein, said front wall having a doorway thereincommunicating with said oven cavity and provided with a door movablebetween open and closed positions with respect to said doorway, a sourceof microwave energy, the dimensions of said oven cavity being related tothe wavelength of the microwave energy of said source so that said ovencavity is non-resonant, an antenna arranged within said oven cavity anddisposed substantially in a plane positioned adjacent to and below andsubstantially parallel with said top wall, said antenna including twosubstantially symmetrical antenna members extending generally parallelto said front and rear walls and having a length equivalent to thewavelength of the microwave energy of said source, and an electricalinput terminal operatively coupled to said source of microwave energyand disposed below the central portion of said top wall between saidantenna members and operatively coupled thereto.

11. The microwave heating oven set forth in claim 10, wherein saidantenna further includes two substantially symmetrical arm structuresdisposed between said electrical input terminal and said antennamembers.

12. A microwave heating oven comprising substantially box-likeconductive wall structure including top and bottom walls and a pair ofside walls and front and rear walls, said wall structure defining anoven cavity therein, said front wall having a doorway thereincommunicating with said oven cavity and provided with a door movablebetween open and closed positions with respect to said doorway, a sourceof microwave energy, the dimensions of said oven cavity being related tothe wavelength of the microwave energy of said source so that said ovencavity is non-resonant, an antenna arranged within said oven cavity and.disposed substantially in a plane position adjacent to and below andsubstantially parallel with said top wall, said antenna including twosubstantially symmetrical antenna members extending between said sidewalls and having lengths equivalent to the wavelength of the microwaveenergy of said source and spaced-apart a distance equivalent to one-halfthe wavelength of the microwave energy of said source, connectingmembers connecting adjacent portions of said antenna members at pointsspaced a distance equivalent to one-quarter wavelength of the microwaveenergy of said source from the adjacent side wall, and an electricalinput member interconnecting the midpoints of said connecting membersand operatively coupled to said source of microwave energy.

13. The microwave heating oven set forth in claim 12, wherein thedistance between said side walls and the distance between said front andrear walls are slightly less than the wavelength of the microwave energyof said source, and means is, provided capacitively loading said antennamembers to impart thereto an electrical length greater than themechanical length thereof so that the electrical length thereof isequivalent to the wavelength of the microwave energy of said source.

14. A microwave heating oven comprising conductive wall structuredefining an oven cavity and having a doorway therein communicating withsaid oven cavity and provided with a door movable between open andclosed positions with respect to said doorway, a source of microwaveenergy adapted to operate at a predetermined wavelength, an antennaarranged within said oven cavity and operatively coupled to said sourceand resonant at said predetermined wavelength, said antenna includingtwo substantially symmetrical arm structures disposed substantiallysymmetrically with respect to the side walls of said oven cavity,whereby said antenna constitutes a dipole antenna, the dimensions ofsaid oven cavity being related to said predetermined wavelength of themicrowave energy of said source so that said oven cavity is free ofresonant modes at said predetermined wavelength.

15. A microwave heating oven comprising substantially box-likeconductive wall structure including top and bottom walls and a pair ofside walls and front and rear walls, said wall structure defining anoven cavity therein, said front wall having a doorway therein connectingwith said oven cavity andprovided with a door movable between open andclosed positions with respect to said doorway, a source of microwaveenergy having a predetermined operating wavelength, an antenna arrangedwithin said oven cavity and disposed substantially in a plane positionadjacent to and below and substantially parallel with said top wall andoperatively coupled to said source, said antenna including twosubstantially symmetrical antenna members extending between said sidewalls, the distance between said side walls and the length of saidantenna members being of the same order of magnitude as the wavelengthof said source but diifering in mechanical length therefrom, thedimensions of said oven cavity being related to said predeterminedwavelength so that said oven cavity is free of resonant modes at saidpredetermined wavelength, and means reactively loading said antennamembers to impart thereto an electrical length equal to the wavelengthof said source to cause said antenna to be resonant at saidpredetermined wavelength.

16. The microwave heating oven set forth in claim 15, wherein thedistance between said side walls is slightly less than the wavelength ofthe microwave energy of said source, and said reactive loading meanscapacitively loads said antenna members to impart thereto an electricallength greater than the mechanical length thereof so that the electricallength thereof is equivalent to the wavelength of the microwave energyof said source.

References Cited by the Examiner UNITED STATES PATENTS 2,937,259 5/1960De Bell 2l910.55

RICHARD M. WOOD, Primary Examiner.

1. A MICROWAVE HEATING OVEN COMPRISING CONDUCTIVE WALL STRUCTUREDEFINING AN OVEN CAVITY AND HAVING A DOORWAY THEREIN COMMUNICATING WITHSAID OVEN CAVITY AND PROVIDED WITH A DOOR MOVABLE BETWEEN OPEN ANDCLOSED POSITIONS WITH RESPECT TO SAID DOORWAY, AN ANTENNA ARRANGEDWITHIN SAID OVER CAVITY, AND A SOURCE OF MICROWAVE ENERGY ADAPTED TOOPERATE AT A PREDETERMINED WAVELENGTH AND OPERATIVELY COUPLED TO SAIDANTENNA, THE DIMENSIONS OF SAID OVEN CAVITY BEING RELATED TO SAIDPREDETERMINED WAVELENGTH OF THE MICROWAVE ENERGY OF SAID SOURCE SO THATSAID OVER CAVITY IS FREE OF RESONANT MODES AT SAID PREDETERMINEDWAVELENTH AND SO THAT THE WAVELENGTHS OF THE RESONANT MODES OF SAID OVENCAVITY ARE DISPLACED BY AT LEAST ABOUT 5% FROM SAID PREDETERMINEDWAVELENGTH.